Central role of heterocellular gap junctional communication in endothelium-dependent relaxations of rabbit arteries

1. The contribution of gap junctions to endothelium-dependent relaxation was investigated in isolated rabbit conduit artery preparations pre-constricted by 10 mu M phenylephrine (PhE). 2. Acetylcholine (ACh) relaxed the thoracic aorta by similar to 60 % and the superior mesenteric artery (SMA) by similar to 90%. A peptide possessing sequence homology with extracellular loop 2 of connexin 43 (Gap 27, 300 mu M) inhibited relaxation by similar to 40 % in both artery types. Gap 27 also attenuated the endothelium-dependent component of the relaxation induced by ATP in thoracic aorta but did not modify force development in response to PhE. 3. N-G-nitro-L-arginine methyl ester (L-NAME, 300 mu M), an inhibitor of NO synthase, attenuated ACh-induced relaxation by similar to 90% in the aorta but only by similar to 40% in SMA (P < 0.05). Residual L-NAME-insensitive relaxations were almost abolished by 300 mu M Gap 27 in aorta and inhibited in a concentration-dependent fashion in SMA (similar to 50 % at 100 mu M and similar to 80 % at 10 mM). Gap 27 similarly attenuated the endothelium-dependent component of L-NAME-insensitive relaxations to ATP in aorta. 4. Responses to cyclopiazonic acid, which stimulates endothelium-dependent relaxation through a receptor-independent mechanism, were also attenuated by Gap 27, whereas this peptide exerted no effect on the NO-mediated relaxation induced by sodium nitroprusside in preparations denuded of endothelium. 5. ACh-induced relaxation of 'sandwich' mounts of aorta or SMA were unaffected by Gap 27 but completely abolished by L-NAME. 6. We conclude that direct heterocellular communication between the endothelium and smooth muscle contributes to endothelium-dependent relaxations evoked by both receptor-dependent and -independent mechanisms. The inhibitory effects of Gap 27 peptide do not involve homocellular communication within the vessel wall or modulation of NO release or action.